The personal computer and other digital devices are rapidly becoming key communication tools for millions of users worldwide. The importance of digital and data network communications has greatly increased with the explosion of the Internet. While electronic mail is still a dominant method of interactive computer communications, electronic conferencing and IP-based telephony are becoming increasingly attractive. The adoption of packet switching and its merging with circuit switching helps drive this communications migration. There are many reasons for this, among them pricing advantages due to improved source utilization, seamless transmissions between monomedia and multimedia communications, as well as between human-to-computer (e.g. web-based) and interpersonal interactions.
According to current scenarios, service and/or bearer parameters can be negotiated with setup and setup response messages. An IP mobile terminal equipment performs a setup signaling according to the Session Initiation Protocol (SIP) used for initiating calls in IP networks, such as the Internet. SIP can be used to establish multimedia sessions or calls such as Internet telephony, multimedia conferencing and distance learning. SIP supports user mobility, that is the ability of end-users to make and receive calls and access subscribed telecommunication services from any location and the ability of the network to track the location of the user. The first step in the initiation of a call using SIP is to locate a SIP server for the callee. Once the SIP server has been found, the client can invite the callee to join a communication session by transmitting an INVITE message. A successful indication consists of an INVITE message followed by an ACK message.
The INVITE message contains a session description that provides the called party with enough information to join the session. If the callee decides to accept the call, it response with a session description of its own. The session description is based on the Session Description Protocol (SDP) which indicates the media components, the transport protocol, the media format, and IP addresses and ports.
Additionally, in the SIP signaling message such as INVITE and its response message bodies it is possible to carry various contents in the informative part for the called party. Examples of such contents are MIME (Multipurpose Internet Mail Extensions) contents. These MIME contents carried in SIP signaling messages could be, for instance, greeting pictures or voice clips that are presented to the called subscriber when the incoming call is alerted to him. Similarly, in SIP messages there are some header fields informative for the called subscriber, such as the subject header. Due to the outband SIP call signaling via a separately routed control channel, the usage of different chargeable actions is enabled even if the call is never connected, e.g. if the initial setup signaling was not successful. Thus, inter-operator accounting based on an accumulation of chargeable signaling information is not possible for the amount of data carried in signaling messages. In present accounting systems, only call amounts and durations are gathered.